This invention generally concerns a digital data processing system capable of reducing transmission time and storage requirements, and particularly concerns compression and subsequent decompression of digital data images, such as X-rays, which are inherently similar in many respects; and, wherefore, a priori knowledge of such images can be used in their compression.
Although the preferred areas of application (and the following discussion of background and description) of the invention pertain mainly to the medical and teleradiological fields, similar problems as discussed herein exist in other similar systems which deal with the compression and storage of large amounts of digital data; accordingly, advantages and benefits of this invention are equally applicable to such other systems. With such an understanding, it should be appreciated that the following discussion of specific exemplary present embodiments is in no way meant to limit the scope of the present invention to strictly medical applications.
Electronic transmission of various types of digital data, in particular radiological data (teleradiology), involves the transmission of a tremendous volume of data. Unless expensive broadband systems are used, such high data volume causes the transmission process to be relatively slow, and with the quality of the transmitted image often deteriorated. In the case of teleradiology, the transmission time is crucial for prompt analysis of a patient's condition and the sharing of analytical expertise between hospitals, clinics, and the like.
State of the art radiological systems produce digital imagery which may contain one megabyte of data per image. Since there is information throughout the frame, systems which attempt to transmit for example chest X-rays must process the entire volume of image data. Because of such large volume of data, most systems include relatively expensive image compression schemes. The efficiency of compression can be measured by the ratio of the necessary storage size before compression to the necessary storage size after compression (compression ratio). However, the compression algorithms incorporated in such schemes rarely achieve compression ratios greater than 5:1. State of the art image transmission systems incorporating conventional image compression and using 9600 baud telephone lines still require more than six minutes for the transmission of one 1024.times.1024 image.
Storaqe of the imaqery data is also a problem. One uncompressed image requires approximately one megabyte of storage space, obviously necessitating considerable storage capability to maintain a library of any significant number of different digital images.
Various forms of digital image differencing for image compression and digital image differencing for image enhancement are known. However, such differencing methods of the prior art do not provide features and advantages of the present invention.
Methods which perform differencing for image enhancement generally aim to enhance images by removing contrast detail information which is similar in the images. In such cases, the information in the difference image is typically presumed to be diagnostically interesting. The difference image contains sharply reduced intensity values in the regions where the contrast detail of the original images was similar, and normal intensity values in the regions where there were appreciable differences. Hence, contrast between such forms of difference information and the background is enhanced. Such prior method of image enhancement is often used in medical imaging to examine changes in the medical condition of specific parts of anatomy or to visualize vessels and cavities by subtracting images taken before and after some tracer element is injected (e.g., Digital Subtraction Angiography).
The difference image obtained with the foregoing often contains less contrast detail than the original images. At a minimum, if the original images were similar, the intensive values of most pixels in the difference image are low. This, however, does not mean that the size of the difference image (i.e., the volume of data) is smaller than that of the original image. Indeed, a difference image which entirely comprises eight bit, low intensity pixels is as large as an original image which contain the same number of eight bit pixels, but which displays maximum contrast detail.
U.S. Pat. No. 3,905,045 to Nickel concerns an apparatus for image processing using a plurality of operations in sequence to produce a difference image, including bilinear mapping of one image on the other to register the images and photoequalization to generate a difference image. Image warp transformation is disclosed using operator selected match points on a pair of images, along with an image correlation process using a second group of matching points. The size of the differenced image is, however, as large as either of the two original images, wherefore no compression is achieved.
U.S. Pat. No. 4,533,947 to Smith discloses a method for increasing the data storage transfer rate in a system to permit the recording of a rapid sequence of high resolution images by eliminating designated portions of each image in the sequence. The disclosed method operates on the premise that two images which are related, particularly through temporal continuity, have similar intensity values over a significant portion of the pixel matrix. Portions of the image where pixel values have not changed beyond some threshold from corresponding portions in a related image are recognized, and those image portions where significant change is indicated are transmitted. A high degree of spatial correlation is assumed between the image matrices before they enter the system. The method does not accommodate situations in which images do not match because of differences such as in orientation, location, gray scale intensity range, and scale of component elements. Such may be typical whenever images are not recorded in rapid sequence.
U.S. Pat. No. 4,742,558 to Ishibashi et al. discloses a method for the reduction of codes necessary to display a designated image area contained in a global image. An original image is represented by a hierarchical structure from a global image of high level to a local image of low level. Only difference information between the images of the respective levels is coded and stored so that the number of codes will not be increased. The method includes a step in which differencing of image data takes place, but one image is not differenced from another relatively unrelated image. Rather, the image is compared to and differenced from itself. The objective is apparently not to reduce image data but to reduce the cost of splitting the image into several levels of a hierarchical structure according to different resolutions of the image.
U.S. Pat. No. 4,802,093 to Ema discloses an X-ray image processing apparatus which performs image differencing and gray scale transformation to reduce the gray scale differences between images prior to differencing. The invention relates to image enhancement by digital differencing such as Digital Subtraction Angiography (DSA). The system transforms the gray scales of the respective images to minimize the total intensity difference between them. This results in a difference image whereby the background to the contrast enhanced areas has lower intensity values, making the contrast enhanced areas more visible.
U.S. Pat. No. 4,809,350 to Shimoni et al. discloses an image compression system which mathematically approximates the image function and defines the difference image as the difference between the image and the approximating model. An image is used to predict itself and the model is subsequently used in the differencing operation. Although steps of acquisition, image subtraction, compression, storage, decompression, and image addition are disclosed, the differencing method is fundamentally different from the present invention. Shimoni et al. is inherently limited as an effective system because, for example, it uses an inherently related image in the subtraction process, while the present invention advantageously makes use of a relatively unrelated reference image. The fact that the images of the present invention are not inherently related allows greater flexibility and higher compression ratios as explained in the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.
U.S. Pat. No. 4,685,146 to Fenster et al. discloses a method for correcting for artifacts generated by object-motion caused misregistration between different images. Enhancement is provided through image differencing, and in particular a method is disclosed for assuring valid registration of images to be compared to obtain image enhancement with minimal artifacts.
U.S. Pat. No. 4,635,293 to Watanabe and U.S. Pat. No. 4,644,582 to Morishita et al. also generally disclose various methods for image registration and alignment.